1. Field of the Invention
This invention relates to a permanent magnet having improved corrosion resistance and to a method for producing the same.
2. Description of the Prior Art
It is known to produce permanent magnets of a rare earth element-iron-boron composition to achieve high energy product at a lower cost than samarium cobalt magnets. These magnets do, however, exhibit severe corrosion by oxidation in air, particularly under humid conditions. This results in degradation of the magnetic properties during use of the magnet.
Efforts have been made to improve the corrosion resistance of these magnets, such as by applying metallic platings thereto, using aluminum-ion vapor deposition coatings, organic resin coatings, synthetic resin coatings, metal-resin double layer coatings, as well as combinations of these coating systems. In addition, chemical surface treatments have been employed with these magnets in an attempt to improve the corrosion resistance thereof.
Metallic platings, applied by electro or electroless plating practices, provide platings of nickel, copper, tin and cobalt. These practices have been somewhat successful in improving the corrosion resistance of these magnets. Problems may result with this plating practice from the acidic or alkaline solutions used in the pretreatment employed prior to the plating operation. These solutions may remain in the porous surface of the magnet or may react with neodymium-rich phases thereof to form unstable compounds. These unstable compounds react during or after plating to cause loss of plating adhesion. With metallic platings, it is common for the plating to exhibit microporosity which tends to accelerate reaction of unstable phases. For example, if there is a reactive media, such as a halide, in the environment, such as is the case with salt water, a galvanic reaction may result between the metallic plating and the unstable phases of the magnet.
With aluminum-ion vapor deposition no pretreatment is required and thus the problems of metallic platings in this regard are avoided. Coatings of this type, however, are characterized by significant microporosity because of the nonuniform deposition of the coating on the surface of the magnet. In addition, this practice is not amenable to mass production processes and thus is too expensive for commercial application.
The use of resin coatings suffer from poor adhesion to result in the gradual removal of the coating followed by oxidation of the magnet surface at the removed coating portion thereof.
Metallic-resin double layered coatings if not applied in a continuous fashion result in accelerated, spreading corrosion from any areas of coating discontinuity.
Chemical surface treatments, including chromic acid, hydrofluoric acid, oxalic acid or phosphate treatments, all suffer from the disadvantage of requiring expensive equipment to comply with environmental regulations. Consequently, these practices are not commercially feasible from the cost standpoint.
All of the conventional methods for improving the corrosion resistance of permanent magnets of this type suffer from the same deficiency in that the corrosion protection is obtained by a surface treatment of the magnet. Accordingly, the magnet per se is not stabilized with respect to corrosion by any of these surface-treatment practices.
It is known to vary the composition of the magnet to improve the corrosion resistance thereof. Alloy modifications of this type are disclosed in Narasimhan et al., U.S. Pat. No. 4,588,439 wherein an oxygen addition is added to improve corrosion resistance by reducing the disintegration of the magnet in humid high-temperature conditions. A. Kim, and J. Jacobson: IEEE Trans on Mag. Mag-23, No. 5, 1987 disclose the addition of aluminum and dysprosium or dysprosium oxide for this purpose. This publication also recognizes that chlorine contamination of the magnet results in deterioration of the corrosion resistance both in humid and in dry air at elevated temperature. Sagawa et al., Japanese Patent No. 63-38555, 1986 disclose the addition of cobalt and aluminum to improve corrosion resistance. These alloying additions are combined with reduced carbon and oxygen contents. Takeshita, and Watanabe: Proceedings of 10th Int'l Workshop on RE magnets and their application (I), Kyoto, Japan, 1989 disclose the addition of oxides of chromium, yttrium, vanadium and aluminum for purposes of corrosion resistance in these alloys. H. Nakamura, A. Fukumo and Yoneyaaama: Proceedings of 10th Int' l Workshop on RE Magnets and Their Application (II) Kyoto, Japan, 1989, discloses the substitution of a portion of iron with cobalt and zirconium for this purpose. A. Hasabe, E. Otsuki and Y. Umetsu: Proceedings of the 10th Int'l Workshop on RE Magnets and their Application (II), Kyoto, Japan, 1989, disclose various anodic polarization techniques for improving corrosion resistance.
All of these practices may result in improved corrosion resistance but otherwise provide problems, such as increased cost or degradation of magnetic properties. For example, the addition of cobalt increases the Curie temperature but causes a decrease in coercive force. The addition of the aforementioned oxides degrades the energy product of the magnets.